<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>for循环优化</title>
</head>
<body>
   <p class="btn">add</p>
   <p class="btn">add</p>
   <p class="btn">add</p>
   <p class="btn">add</p>
   <p class="btn">add</p>
   
   <p class="btn">add</p>
   <p class="btn">add</p>
   <p class="btn">add</p>
   <p class="btn">add</p>
   <p class="btn">add</p>
   <p class="btn">add</p>
	<script>
		var aBtns = document.getElementsByClassName('btn');
		for(var i = 0 ; i < aBtns.length ; i++){
			console.log(i)
		}
		for(var i = 0, len = aBtns.length;i < len;i++){
			console.log(i)
		}
	</script>
</body>
</html>